Positron Emission Tomography (PET) imaging provides high resolution and quantitation from the PET images. Peptides or other small molecules can be labeled with the positron emitters 18F, 64Cu, 66Ga, 68Ga, 76Br, 94mTc, 86Y, and 124I to name a few. The positron emitted from the nucleus of the isotope is ejected with different energies depending on the isotope used. When the positron reacts with an electron two 511 keV gamma rays are emitted in opposite directions. The energy of the ejected positron controls the average distance that a positron travels before it is annihilated by hitting an electron. The higher the ejection energy the further the positron travels before the collision with an electron. A low ejection energy for a PET isotope is desirable to minimize the distance that the positron travels from the target site before it generates the two 511 keV gamma rays that are imaged by the PET camera. Many isotopes that emit positrons also have other emissions such as gamma rays, alpha particles or beta particles in their decay chain. It is desirable to have a PET isotope that is a pure positron emitter so that any dosimetry problems will be minimized.
The half-life of the isotope is also important, since the half-life must be long enough to attach the isotope to a targeting molecule, analyze the product, inject it into the patient, allow the product to localize, clear from non-target tissues and then image. If the half-life is too long the specific activity may not be high enough to obtain enough photons for a clear image and if it is too short the time needed for manufacturing, commercial distribution and biodistribution may not be sufficient. F-18 (β+635 keV 97%, t1/2 110 min) is one of the most widely used PET emitting isotopes because of its low positron emission energy, lack of side emissions and suitable half-life. The F-18 is produced with a high specific activity. If the F-18 is attached to a molecule which has a very high uptake such as 2-fluoro-2-deoxy glucose (FDG) then specific activity is not as important. However, if one is targeting a receptor with a labeled peptide or performing an immunoPET pretargeting study then the specific activity is important.
Conventional F-18 labeling of peptides involves the labeling of a reagent at high specific activity and then conjugation of the F-18 labeled reagent to the peptide. An example is the labeling method of Poethko et al. (J. Nucl. Med. 2004; 45: 892-902) in which 4-[18F]fluorobenzaldehyde is first synthesized and purified (Wilson et al, J. Labeled Compounds and Radiopharm. 1990; XXVIII: 1189-1199) and then conjugated to the peptide. The peptide conjugate is then purified by HPLC to remove excess peptide that was used to drive the conjugation to completion. The two reactions and purification would not be a problem if F-18 had a long half-life. However the half-life of F-18 is only 2 hr so all of the manipulations that are needed to attach the F-18 to the peptide are a significant burden.
These methods are tedious to perform and require the use of equipment designed specifically to produce the labeled product and/or the efforts of specialized professional chemists. They are not kit formulations that could routinely be used in a clinical setting.